Atomic vacancy

An account of the mechanism for creep in solids placed under a compressive hydrostatic suess which involves atom-vacancy diffusion only is considered in Nabano and Hemirg s (1950) volume diffusion model. The counter-movement of atoms and vacancies tends to relieve the effects of applied pressure, causing extension normal to the applied sU ess, and sluinkage in the direction of the applied sU ess, as might be anticipated from Le Chatelier s principle. The opposite movement occurs in the case of a tensile sU ess. The analysis yields the relationship... 

Because of die rigidity and directionality of die covalent bonds die energies of self-diffusion have been found to be higher diaii diose of metals. In die case of silicon, it appears drat a furdier complication is drat an intersti-tialcy mechanism predominates above 1000°C. Below diis teiiiperamre, bodi elements appear to self-diffuse by atom-vacancy exchange as for die metals. 

Another example of the use of neutron diffraction to understand the role of atomic vacancies in producing a superconducting metal oxide phase is work that has been performed on Bao Kq 4fii03. This work demonstrates that at the synthesis temperature (700° C), under the proper conditions, oxygen vacancies are created to allow the formation of the parent phase with bismuth largely in the +3 oxidation state. The presence of the vacancies allows the incorporation of potassium in the... 

Unlike the carbides, they tolerate metal-atom vacancies. 

Interchange of Atoms Vacancy Hopping Interstitial Movement Dissociative Exchange... 

The anodic dissolution of metals on surfaces without defects occurs in the half-crystal positions. Similarly to nucleation, the dissolution of metals involves the formation of empty nuclei (atomic vacancies). Screw dislocations have the same significance. Dissolution often leads to the formation of continuous crystal faces with lower Miller indices on the metal. This process, termed facetting, forms the basis of metallographic etching. 

The D band, the disorder induced mode, normally appears between 1250 and 1450 cm. This band is activated in the first-order scattering process of sp2 carbons by the presence of in-plane substitutional hetero-atoms, vacancies, grain boundaries or other defects and by finite-size effects [134], The G band is the second-order overtone of the D band. 

Figure 9.27 A (N-V) center in diamond, consisting of a carbon atom vacancy and a neighboring nitrogen atom impurity.

Let us consider the typical mechanisms of spontaneous processes that decrease /. The direction and driving force of such mechanisms are determined by the laws of equilibrium thermodynamics, and the rate is proportional to diffusion in gases, viscosity in liquids, and transfer of atoms, vacancies, and other defects in solids. 

The effect of atomic motion in the solid state on nuclear resonance line width is illustrated by the behavior of Na resonance from NaCl as a function of temperature 97). In Fig. 9 is shown the variation of the Na line width with temperature for pure NaCl and NaCl doped with an atomic fraction concentration of 6 X 10 of CdCU. As discussed in Section II,A,2 the low-temperature, rigid-lattice line width will narrow when the frequency of motion of the nuclei under observation equals the line width expressed in sec.-. The number of vacancies present should be equal to the concentration of divalent impurities and the jump frequency of Na+ is the product of the atomic vacancy concentration and the vacancy jump frequency... 

Fig. 3. Splitted view of atoms along the c axis of the hexagonal structure showing the two possible fluoride ion locations. In stoichiometric fluorapatite, fluoride ions locate in the equilateral triangle formed by Ca(ll) ions. In type B carbonate apatite, the replacement of P04 ions by ions creates an oxygen atom vacancy which may be occupied by a second kind of fluoride ion (adapted from Ref. [4]). (See Colour Plate Section at the end of this book.)...

The introduction of Fca-atom vacancies modifies the energy diagram of Fig. 3. The preservation of local charge neutrality requires that each Fea-atom vacancy have five of its six nearest neighbors as Fee ions, which means that the ai(, ) states of five Fea ions neighboring a vacancy, , are raised an energy Ea above the Fermi energy. They become acceptor states inaccessible to the mobile electrons at low temperatures. It is possible to express this situation with the structural formula... 

The simplest lattice defects as far as FIM observations are concerned are point defects, such as vacancies, self-interstitials and substitutional as well as interstitial impurity atoms. Vacancies invariably show up as dark spots in the field ion images. Other point defects may appear as either bright image spots or vacancies in the image. Thus these defects can be identified from field ion images of high index planes where all the atoms in a plane are fully resolved. 

The ability of the STM to achieve atom-resolved real-space images of localized regions of a surface and to directly resolve the local atomic-scale structure has provided essential insight into the active sites on catalysts and emphasized the importance of edges, kinks, atom vacancies, and other defects, which often are difficult to detect with other techniques (46-49). It is evident, however, that STM cannot be used to image real catalysts supported on high-surface-area, porous oxide carriers. 

The 1-2-3 superconductor has a perovskile-like structure (7.33a,c). There are systematic oxygen atom vacancies in the unit cell compared to a stack of simple perovskite unit cell (Fig. 7.33b). These occur between adjacent copper atoms in the chains along the c axis. The vacancies are in the yttrium atom plane. There are also vacancies between copper atoms along the a axis in the copper-and-oxygen planes... 

Fig.6. Relationship between structure vacancies and Si(3Si, 10H) sites (O) Si, ( ) trivalent atom, ( ) vacancy...

This fraction is determined by the step-dance between a specified vacancy and the (tagged) atom during their encounter, which does not end before the atom-vacancy pair has definitely separated. Normally, a new and independently moving vacancy comes along much later and begins the next encounter with the tagged atom. 

Figure 8.3 Atom-vacancy exchange in f.c.c. crystal. Atom initially at A jumps into a...

Equation 8.19 contains the correlation factor, f, which in this case is not unity since the self-diffusion of tracer atoms by the vacancy mechanism involves correlation. Correlation is present because the jumping sequence of each tracer atom produced by atom-vacancy exchanges is not a random walk. This may be seen by... 

The analysis conducted in this Chapter dealing with different theoretical approaches to the kinetics of accumulation of the Frenkel defects in irradiated solids (the bimolecular A + B —> 0 reaction with a permanent particle source) with account taken of many-particle effects has shown that all the theories confirm the effect of low-temperature radiation-stimulated aggregation of similar neutral defects and its substantial influence on the spatial distribution of defects and their concentration at saturation in the region of large radiation doses. The aggregation effect must be taken into account in a quantitative analysis of the experimental curves of the low-temperature kinetics of accumulation of the Frenkel defects in crystals of the most varied nature - from metals to wide-gap insulators it is universal, and does not depend on the micro-mechanism of recombination of dissimilar defects - whether by annihilation of atom-vacancy pairs (in metals) or tunnelling recombination (charge transfer) in insulators. 

The corrosion reactions cause another serious problem. They produce a number of surface reaction iniermediates such as dangling bonds and atomic vacancies, having electronic levels within the band gap. Namely, corrosion reactions produce a number of mid-gap surface states, which act as surface recombination centers for photogenerated carriers. The Voc is decreased largely by the production of such surface states.3,41... 

In summary the results show that (1) neither of the nitrided phases contains sulphur (2) the observed nitrogen contents, in both cases, are higher than the theoretical value in MoN, with the most significant difference occurring in L-MoN, as already noted. Note that these results were collected after treatment of the powders at 573 K under hydrogen, in order to remove eventual adsorbed species and (3) the low density values comparatively to the dXRD density tabulated for 5-MoN imply molybdenum atom vacancies, not an excess of N atoms. 

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